"Converting a varbinary to a varchar" can mean different things.
If the varbinary is the binary representation of a string in SQL Server (for example returned by casting to varbinary directly or from the DecryptByPassPhrase or DECOMPRESS functions) you can just CAST it
declare @b varbinary(max)
set @b = 0x5468697320697320612074657374
select cast(@b as varchar(max)) /*Returns "This is a test"*/
This is the equivalent of using CONVERT with a style parameter of 0.
CONVERT(varchar(max), @b, 0)
Other style parameters are available with CONVERT for different requirements as noted in other answers.